Aluminide coatings are widely used in the gas turbine engine industry to provide protection against oxidation and corrosion degradation to superalloy articles used in the engine. U.S. Patents which are indicative of the skill in the art relative to aluminide coatings include the following: U.S. Pat. Nos. 3,079,276, 3,276,903, 3,667,985, 3,801,353, 3,837,901, 3,958,047, 4,132,816, 4,142,023, 4,148,275 and 4,332,843. In general, aluminide coatings are formed by heating a powder mixture containing a source of aluminum, an activator, and an inert diluent in the presence of the article to be coated. The article may either be embedded in the powder mixture (and the process is termed a "pack cementation" process) or the article is in out-of-contact relation with the powder mixture (and the process is termed a "gas phase" process).
The source of aluminum may be pure aluminum metal or it may be an alloy of aluminum such as Co.sub.2 Al.sub.5, as disclosed in U.S. Pat. No. 4,132,816 to Benden et al; U.S. Pat. No. 3,958,047 to Baldi discloses the use of Ni.sub.3 Al as the source of aluminum; and U.S. Pat. No. 4,332,843 to Ahuja discloses the use of Fe.sub.2 Al.sub.5. Activators which have been used in the aluminiding process generally include halides of alkali or alkaline earth metals. See, e.g., the aforementioned patent to Benden. Aluminum oxide is typically added to the powder mixture as a buffer or diluent, in order to control the aluminum activity of the mixture. There are also references in the prior art that aluminum oxide prevents the powder mixture from sintering together during the coating process. See, e.g., U.S. Pat. No. 3,667,985 to Levine et al.
U.S. Pat. No. 3,794,511 to Baranow discloes that a nickel alloy having an aluminide coating which contains Misch metal has better resistance to sulfidation degradation than the same alloy with an aluminide coating containing no Misch metal. The coatings are produced by heating the article in an aluminum-Misch metal alloy powder, the alloy containing between 27 and 45 weight percent aluminum.
In U.S. Pat. No. 3,996,021 to Chang et al, a small amount of hafnium is added to a powder mixture containing an Al-Ti-C alloy, an activator such as NH.sub.4 F, and aluminum oxide. The powder mixture is said to produce an aluminide coating containing between 0.1 and 10 weight percent hafnium.
U.S. Pat. No. 3,993,454 to Giggins et al suggests that coatings which contain hafnium have better high temperature properties (e.g., resistance to oxidation and corrosion) than do coatings containing yttrium.
Other patents which relate to diffusion coatings include U.S. Pat. Nos. 2,801,187, 3,625,750, 4,123,595 and 4,156,042; U.S. Pat. No. Reissue 26,001; and Japanese Patent No. 55-82769.
Notwithstanding the developments in aluminide coatings, engineers still expend considerable effort in an attempt to develop coatings having improved properties.